If you were to ask people what influences the abundance of fish in the sea, most would probably answer ‘commercial fishing’. While it is true that
commercial fishing can deplete fish stocks, another important factor,
the ‘environment’, or more accurately, environmental variability,
is often overlooked as a determinant of fish abundance.

Atlantic cod (Gadus morhua)

Certainly,
when the environment is benign, commercial fishing, referred to as a
‘top-down’ control of fish abundance (think of fishing as a form
of predation) can be the most important influence. However, when the
environment changes and becomes unfavourable, the environment will
become a key driver of fish abundance, and this is referred to as
‘bottom-up’ control. Understanding the interplay of bottom-up and
top-down controls is vital for sustainable fisheries, and especially
at a time of climate change and warming seas.

Stylised diagram of the plankton food web with respect to the herring, a fish that feeds upon plankton throughout its life. The arrows show the interconnections in the food web, ‘who eats who’, revealing the complexity and how it might easily uncouple.

The plankton live at
the sea surface and their habitat is likely to warm due to
anthropogenic climate change. More and more studies are now providing
evidence that the plankton’s distribution, abundance and
seasonality is altering as their habitat warms, uncoupling the marine
food chain. In the North Atlantic ocean many of these studies have
focused upon understanding the population dynamics of the cod Gadus
morhua due to its commercial importance and history of population
declines.In the Northeast Atlantic, warm-temperate, pseudo-oceanic species of copepod have moved northwards by about 10° of latitudes over 48 years between 1958 and 2005 (52–62°N;10°W) as the sea surface has warmed, which is a poleward movement of 23.16 km per year (http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2486.2009.01848.x/full). Cold water species of
copepod have retracted towards the poles and warm-water species have
moved northwards. This movement of copepods has resulted in a 60%
reduction in the preferred food of larval cod in the North Sea, the
cold water copepod Calanus finmarchicus, affecting cod
recruitment (the number of juvenile cod that survive to become
adults).

The thermal niche of the Atlantic cod (blue area) based on mean annual sea surface temperature (SST) during the period 1960 to 2005 and the probability of cod occurrence. Both observed (1960 to 2005, shaded bars, white text and arrows) and projected (based upon climate change scenario A2, 1990 to 2100, black text and arrows) ranges in SST are shown for Iceland (solid vertical lines) and the North Sea (dashed vertical lines), indicating that under climate change scenario A2, the North Sea becomes too warm for high numbers of cod such that it may be an unviable fishery.

So, not only are cod in the North Sea experiencing fishing
pressure, but the warming environment is now exerting bottom-up
control too, both through the food web and upon cod directly
(http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2486.2009.02063.x/full).
You might well argue that cod will just move northwards and so ‘all
will be OK’. Unfortunately, it is not so simple as both the habitat
(bathymetry) and temperature must be suitable. The North Sea is a
shallow, nutrient rich sea and so supports a productive plankton food
web. (Shallow seas, like nutrient rich upwelling regions, support the
world’s most productive fisheries.) Northwards of the North Sea the
ocean deepens and is less favourable for plankton and cod. Here, the
shallow seas are restricted to continental shelves along the coast of
Norway and surrounding Iceland. The next, large, favourable habitat
for cod is the Barents Sea.

Cod feed upon crabs,
lobsters and shrimps and in regions where cod have declined through
overfishing, such as in the Northwest Atlantic, there has been a
large increase in these decapods, which may be due to decreased
predation pressure upon them (relaxation of a top-down control). In
the North Sea, where cod have declined due to the combined effects of
fishing and environmental change, decapods have also increased in
abundance. Currently, the abundance of decapods in the North Sea is
also influenced positively by warming; they produce more offspring
when the sea is warmer and warm-water species have also invaded. And so, in
the North Sea, the decline of cod and the warming environment may
both be favouring decapods. In turn, this has ramifications for other
species and the ecology of the North Sea; there are ‘winners and
losers’ in this ecosystem
(http://blueplanetsociety.blogspot.co.uk/2015/11/plankton-as-indicators-of-ecosystem.html).

Two recent studies of
cod have again shed light upon how important the environment is as a
driver of an animal’s abundance. These two studies are focused at
the northern and southern limits of the distribution of cod in the
Northwest Atlantic. Here, at the species’ southern limit in the
Gulf of Maine, warming seas are reducing the abundance of cod
(http://www.sciencemag.org/content/early/2015/10/28/science.aac9819.full).
In contrast, at the northern, cold boundary of the species’
distribution in Newfoundland, warming seas are having a positive
impact upon their numbers through the food web
(http://www.nrcresearchpress.com/doi/10.1139/cjfas-2015-0346#.VjjRWqS3KPQ).
Interestingly, these two studies also reveal that environment is a
more important determinant of abundance than controls upon
overfishing.

If the global climate
and the sea surface continues to warm it does not mean that we will
not experience some seasons and years that are colder than others.
(Of course, due to the warmer baseline temperature, these colder
years will not be as cold as they might have been in the past.)
Again, using cod as an example, in these cooler years we may see an
increase in cod abundance among populations that reside at the warmer
edge of the species’ niche due to more favourable conditions (such
as cooler conditions would create in the Gulf of Maine or the North
Sea). In these circumstances, if we only consider commercial fishing
activity to influence abundance, we may be lulled into a false belief
that a fish stock is recovering due to effective fishery management
strategies, only to find that we were wrong when the sea temperature
increases again.

Ecosystems by their
nature are complex with many linkages among the species they contain.
Understanding the interactions among species, and how and why they
change, which must include an understanding of the environment, is
key to their sustainable exploitation. Consideration of environmental
changes is absolutely necessary with regard to anthropogenic climate
change. While there is no guarantee that setting quotas will enable a
stock to resist adverse climatic conditions, an absence of regulation
might well precipitate a stock’s collapse, or might cancel any
short-term benefit of improved environmental conditions.